Thermosetting polymer products of various types have been produced by various processes and apparatus ranging from free mold pouring of resin with fillers to double enclosed molds where the polymer material is heated in the mold and after polymerization, the mold is opened to produce a finished molded product.
Thermosetting resins in the form of kitchen sinks, vanity tops, counter tops, and the like, have been produced for many years using various types of molds and processes. Typically, the thermosetting polymer, such as a polyester, or a polyacrylate, is formed in a mold, and cured in the mold to produce the finished product. A problem with this technique is that the thermosetting resins tend to shrink as they cure. Moreover, during the shrinking process, while the curing reaction proceeds to completion, latent stresses are created in the finished product. These latent stresses cause present or future microscopic fissures in the body and surfaces of the finished thermoset products. These latent stresses and microscopic fissures and crazes cause a problem when the cast products are used as kitchen sinks, vanity tops, countertops and the like, wherein the surfaces are exposed to mild caustic and acidic cleaning chemicals, in the form of surface cleaning compounds, detergents, bleaches and polishes.
Another problem, particularly prevalent with kitchen sinks, and the like, is that the thermoset material is subjected to sudden thermal shock (sudden contraction or expansion) caused by exposure to hot or cold water. The microfissures become aggravated under such treatment, and eventually enlarge to the point that visible cracks occur. Eventually, the fixture must be replaced.
The American National Standard for Plastic Laboratories (ANSI) has a number of standard tests which must be passed in order for a particular lavatory product to be commercially acceptable. One test, ANSI Z124.3, Section 6.3, requires, among other things, that the lavatory unit be subjected to thermal shock exposure by having the product endure at least 500 cycles of hot and cold water. Each cycle consists of 1.5 minutes of 150.degree. F. water flowing at a rate of 1 gal./minute, followed by a 30 second drain. The product is then exposed to 1.5 minutes of 50.degree. F. water flowing at 1 gal./minute. No crazing, cracking, blistering or spalling must be visible in the product at the conclusion of 500 cycles.
German Patent No. 29 22 675, Karl Schock, issued Jan. 29, 1981, discloses a process of making articles of mineral filled acrylic resin. The articles include kitchen sinks, vanity tops and a variety of similar products. In the Schock process, it is mentioned that the material is poured into a closed mold. During the polymerization process, the polymer shrinks. To accommodate this, a force is applied to one component of the mold to compensate for the shrinkage of the polymer material as it cures. The material is polymerized to completion in the mold, after which the mold is opened and the finished cured product is removed.
German Patent No. 371295 A1 describes a process whereby a first layer of polymer material is poured into a mold as first coating. At a later stage, the same or another polymer material is poured into the mold as reinforcement.
U.S. Pat. No. 4,652,596, Williams et al., granted Mar. 24, 1987, discloses a process for producing cast unsaturated thermosetting resins which are purportedly highly stain resistant. A pressurization step is applied to the thermosetting resin prior to casting to reincorporate any monomer which has volatilized during a previous vacuum deaeration step. The resultant casting is non-porous in cross-section which produces a product which is both stain resistant and machinable in the field without exposing voids.
U.S. Pat. No. 4,473,673, granted Sep. 25, 1984, Williams et al., describes a method in which basic resins are mixed with mineral filler in a vacuum process to densify the mixture before it is poured into a mold. The mold is then vibrated to accommodate the material and avoid air entrapments.